FIELD OF THE INVENTION
The present invention relates to a process for separating organic acids from aqueous solutions. In particular, the invention relates to a process for purifying the process water in aldol condensation reactions.
Carboxylic acids are formed as byproducts in many base-catalyzed condensation reactions of organic compounds, in particular in the reaction of aldehydes. For example, the condensation of n-butyraldehyde to form ethylhex-2-enal and water is a process of great industrial importance. In this process, the reaction mixture is usually worked up as follows: in the first step, the product-containing organic phase is separated from the aqueous phase (catalyst solution and reaction water) in a settling tank. Owing to the polarity of the organic acids formed in the side reactions and the base used as catalyst, these two components are principally present in the aqueous phase. Direct feeding of this aqueous phase to the effluent treatment plant is therefore usually undesirable, owing to the high loading with bases and organic constituents. In addition, on account of the injection of the base with the aqueous phase from the process, to ensure a constant catalyst concentration, fresh base must constantly be fed to process, which is cost-intensive.
WO 92107648 discloses a process for the recovery of sodium hydroxide solution from aldol condensation process waters. Cation-exchange membranes are used in this process, the object of which is to treat the aqueous phases arising in the aldol condensation. In addition to the basic catalyst, the aqueous phase in this process also comprises the metal salts of the organic acid and alcohol formed as byproducts. This solution is conducted through the anode chamber into a two-chamber membrane electrolysis cell. When an electric field is applied, the metal cations migrate into the cathode chamber and react with the hydroxide ions formed at the cathode to give the metal hydroxide. In the anode circuit, an aqueous solution remains which comprises predominantly only the organic acids and alcohol. Disposal of this solution is complex, since the organic constituents are only present at low concentrations, and the solution cannot thus be fed directly to thermal waste disposal. Recovery of the organic acids present in low concentrations is also not worthwhile.
DE 196 04 903 discloses a similar process for separating an alkali solution from the product arising in the aldol condensation, which process operates according to the principle of three-chamber electrolysis or three-chamber electrodialysis. In this process, the product-containing phase, after removal of an aqueous phase, is conducted through the central chamber of the electrolysis cell which is bounded by an anion-exchange membrane and a cation-exchange membrane. In the case where the catalyst is a sodium hydroxide solution, the Na.sup.+ ions migrate through the cation-exchange membrane in the electric field, and form a sodium hydroxide solution in the cathode chamber, with the OH.sup.- ions produced at the cathode. The OH.sup.- ions present in the central chamber migrate through the anion-exchange membrane into the anode chamber and react with the H.sup.+ ions produced at the anode to form water. In this process for separating alkali solution, the base is removed from the product, but the basic and organic constituents present in the aqueous phase are not worked up.
In both of the above-mentioned processes, the phase to be treated, during the course of the electrolysis or electrodialysis, is depleted in dissolved metal hydroxide, and the conductivity of the solution decreases with advancing electrolysis or electrodialysis. This means that an ever increasing power input is necessary to separate constituents still present in the phase to be treated.
Another known method for treating aqueous solutions comprising organic constituents is acidifying the aqueous solution until phase separation results. However, in this case only the high boilers are removed from the solution, and the organic acids remain in the aqueous phase. In addition, there is further salting of the mass stream of the aqueous solution, which makes disposal still more difficult.
Therefore, the object underlying the invention is to provide a simple process for separating organic acids from aqueous solutions or aqueous waste streams which, in particular, facilitates the workup of aqueous phases separated from aldol condensation reactions.